Many electronic devices utilizing low power converters typically have no on/off switch and are frequently left permanently “plugged-in” to a wall socket. The “standby” power consumption, e.g., when the equipment is off and the batteries are fully charged, of low power converters places a significant load on the public electricity supply. Such concerns have prompted the European Commission Code of Conduct on Efficiency of External Power Supplies, for example, to commit signatories thereto to achieve a no-load power consumption maximum of 300 mW by 2005. Environmental and economic considerations therefore make it desirable to significantly reduce the standby power of low power converters. In addition to reducing the standby power, it is also desirable to maintain good load regulation for the power converter such that the difference between the output voltage at standby and at full-load is minimized.
Conventional low power converters designed for open-loop operation typically exhibit poor output voltage regulation and significantly high power losses at no-load. In order to overcome this drawback, integrated circuit solutions have been implemented for prior art converters to improve output voltage load regulation and to lower standby power consumption. FIG. 1 illustrates a prior art power converter 10 designed for open-loop operation. An input AC voltage from an external AC source (not shown) is typically transformed through a diode bridge 8 into DC power at input terminals 12 and 14. As seen in FIG. 1, power converter 10 is a DC-DC ringing choke converter (hereinafter referred to as an RCC). As is well known in the art, in a conventional RCC, a main switch is connected to a primary winding of a transformer and an output is supplied to the secondary winding through self-oscillation. At a rated load, the conduction time of the main switch is prolonged so as to provide a fixed load current. In a standby lower power or no-load condition, the required load current is minimal, so conventionally, the frequency of the main switch is increased in order to shorten the conduction time accordingly. As the switching frequency for the main switch is increased, however, the switching loss for the power converter also increases. As a result, there is an undesirably high no-load standby power consumption for the conventional power converter.
As seen in FIG. 1, the power converter 10 includes a transformer 56 having a primary winding 66, a secondary winding 68, and an auxiliary winding 20. One end of a primary winding 66 is connected to the input terminal 12 to which the input DC voltage is coupled. The other end of the primary winding 66 is connected to the collector of a main switch 46. Main switch 46 is typically an NPN transistor, as shown in FIG. 1. Alternatively, main switch 46 may be a MOSFET or other suitable switching element. The emitter of main switch 46 is coupled through a resistor 44 to negative input terminal 14. The output of secondary winding 68 is connected to the output terminals 62 and 64 of converter 10 via a rectifying/smoothing circuit that includes a diode 58 and a capacitor 60 connected in a conventional way.
In operation, energy is stored in primary winding 66 during the on time of main switch 46. The on time of main switch 46 is controlled by the signal coupled to the base of main switch 46, i.e., its control input. The auxiliary winding 20 has the same polarity as primary winding 66. One end of auxiliary winding 20 is connected to negative input terminal 14. A resistor 22 is connected in series with a capacitor 24 between the base of main switch 46 and the other end of auxiliary winding 20.
A switch 30 is coupled between input terminal 14 and the control input of main switch 46. Switch 30 is typically a NPN transistor having an emitter connected to input terminal 14. A capacitor 32 is connected across the base and emitter of transistor 30. A resistor 42 is connected in series between the base of transistor 30 and the junction of a resistor 44 and the emitter of main switch 46. A capacitor 36 is connected in parallel with a resistor 38, between a node 75 and input terminal 14. A resistor 26 is connected in series with a diode 34 between node 75 and the end of auxiliary winding 20. The diode 34 has an anode connected to node 75 and a cathode connected to resistor 26. A zener diode 40 has an anode connected to node 75 and a cathode connected to the base of main switch 46. The power converter 10 includes a starting resistor 18 connected in series between input terminal 12 and the control input of main switch 46.
In operation, at start up of converter 10, the input DC voltage provided at the input terminals charges capacitor 16 and a current results through starting resistor 18 that charges the base of main switch 46. When the voltage difference between the base and emitter (Vbe) of main switch 46 exceeds a predetermined threshold, typically 0.6V, main switch 46 is switched on. As a result, the current through main switch 46 is coupled to primary winding 66, where energy is stored.
The conduction state of main switch 46 causes a voltage to develop across resistor 44. The voltage signal across resistor 44 is coupled to the base of switch 30. Switch 30 is switched into a conduction state when there is sufficient charge at the base. The conduction state of switch 30 causes switch 46 to turn off. As a result, the energy from primary winding 66 is transferred to secondary winding 68 and auxiliary winding 20 is charged. The energy from secondary winding 68 is coupled, through the rectifying/smoothing circuit formed by diode 58 and capacitor 60, to output terminals 62, 64. When the energy from secondary winding 68 is depleted, the voltage across auxiliary winding 20 reverses direction. Switch 46 is turned on through build-up of the potential across auxiliary winding 20, enabling the cycle to repeat.
At a low-load or no-load condition for converter 10, the control by switch 30 of the switching of main switch 46 diminishes. For the operation of converter 10 under a no-load condition, energy conversion is done by the switching action of main switch 46 driven by the collapse and build-up of the voltage potential across auxiliary winding 20. The output voltage, Vo, at a no-load level is limited approximately in accordance with the following formula:Vo=(Ns/Nc)*(Vz40+Vbe)where Ns is the number of turns of secondary winding 68, Nc is the number of turns of auxiliary winding 20, and Vz40 is the voltage across zener diode 40. Zener diode 40 draws a portion of the supply current of auxiliary winding 20 in order to control the base current of main switch 46, thereby controlling its turn on time (Ton) in order to limit Vo more specifically according to the following formula expressing V0 as a function of the turn on and turn off time:Vo=(Ns/Np)*Vin*(Ton/Toff)where Vin is the input DC voltage, Np is the number of turns of primary winding 66, and Toff is the turn off time of main switch 46.
Main switch 46 starts to turn off as the voltage difference between the voltage across zener diode 40 and the voltage across capacitor 36 falls below the proper Vbe level for main switch 46. At a no-load condition, faster switching occurs since dt=Lp(di)/Vin with very small (di); where Lp is the inductance of the primary winding 66. The higher the switching frequency of main switch 46, the higher the switching loss for power converter 10. As a result, the no-load standby power consumption for power converter 10 is significantly high, above the European Commission standard of 300 mW referenced above. In addition, since the switching of main switch 46 is uninterrupted for power converter 10, the output voltage, Vo, continues rising as the load is decreased since zener diode 40 will be more accurate due to the decreased voltage spikes at auxiliary winding 20. As a result, there is a large difference between the output voltage at no-load and at full-load for power converter 10. Thus, in addition to the substantial power consumption at standby or no-load, power converter 10 exhibits poor voltage regulation.
A need therefore exists for a circuit and corresponding method for reducing standby power and improving no-load to full-load regulation for ringing choke converters. There is especially a need such a circuit and method in open-loop systems. There is also a need for a circuit for providing reduced standby power and improved regulation through the use of simple, less costly discrete components.